According to the Aberdeen Bestiary (as well earlier texts such as The Physiologus, the Caladrius is a snow-white bird that lives in kings' houses.
Features
If it looks into the face of a sick man, it means that he will live, but if the caladrius looks away, the sick man will die of his illness. To cure the sick man, the caladrius looks at him, and drawing the sickness into itself, the bird fly away, dispersing the sickness and healing both itself and the sick person.
Symbol
The caladrius represents Christ, who is pure white without a trace of blackness of sin. Because the Jews did not believe, Christ turned his face from them and toward the Gentiles, taking away and carrying sins to the cross. Christ turns away from the unrepentant and casts them off; but those to whom he turns his face, he makes whole again.
Theories
There are numerous theories as to where the legend of the Caladrius was started. One of them would be that it was created purely as an analogy of Jesus.
Another is that the Caladrius is based on a real bird. According to the descriptions of its being completely white with no black on it, it is possible that it was based on the dove, or possibly some sort of water bird such as the [[heron] or the white wagtail, which have markings on their heads that resemble a skull, and are regarded in Ireland as mystical. According to T.H. White, few bird dealers would display the Caladrius because people would come in to see if the bird looked away from them, and then just leave the store without buying anything..
Art/Fiction
The caladrius sometimes appears in heraldry; e.g. as the crest of Keith William James.[1]